The FBI has used secret evidence obtained under the Foreign Intelligence Surveillance Act to prosecute at least 27 accused terrorists since 2007, according to a Reuters review of public records.
While the recent spotlight has been on the use of the FISA law by the US National Security Agency for surveillance programs following disclosures by former NSA contractor Edward Snowden, the FBI also makes extensive use of the law for domestic counterterrorism.
The Reuters review highlights the extent to which the FBI has come to rely on FISA to investigate or thwart domestic attacks.
It involved searching the national court docket using the database of Westlaw, which is owned by Thomson Reuters Corp, and includes only cases where prosecutors are required to file a notice under FISA. Other cases where FISA was used may be sealed.
The 27 cases in which the Federal Bureau of Investigation has used FISA evidence include both well-publicized and less-known investigations.
They range from mass murder charges against Army psychiatrist Nidal Hasan for the shootings of 13 people at Fort Hood, Texas, in 2009, to the arrest in April of an 18-year-old in Chicago accused of planning to join an al Qaeda-linked group fighting in Syria. Both men await trial.
In an effort to shore up support for the NSA program, US spy agencies may disclose publicly, as early as Tuesday, for the first time a list of at least 25 terrorist attacks they say were thwarted by the agency's once-secret surveillance operations. Many, if not all, of those NSA operations also used FISA for intelligence gathering.
When the FBI uses FISA, it seeks approval from judges at the secret US Foreign Intelligence Surveillance Court (FISC) for phone, email and electronic surveillance and for searches of property, including "sneak-and-peak" search warrants in which agents covertly enter a business or home when the occupants are away, and try to leave no trail of their visit.
The public court records, often little more than a one-page notification by a Justice Department attorney, provide no specific details of these covert operations. Some case files include defense challenges to the FISA law; none have been successful.
The court files show that the FBI used FISA warrants in recent cases against an Oregon man charged with aiding a Pakistani suicide bomber; a Philadelphia man accused of joining an Uzbekistan terrorist organization; and two Somali-born Minnesota women convicted of raising funds for al Qaeda-affiliated al-Shabaab rebels.
They also include FBI investigations of the New York founder of a radical Islamic website and a Moroccan man convicted of plotting a suicide attack at the US Capitol.
An FBI spokeswoman referred questions about the bureau's use of FISA to the Justice Department, and a spokesman there declined to comment.
FISA warrants are issued by the FISC in Washington. It was created in 1978 following congressional hearings that exposed illegal surveillance of US citizens - without court-authorized warrants.
The court includes 11 judges, all of whom are veteran federal judges at the trial court level. They are appointed by the chief justice of the US Supreme Court to seven-year terms.